The invention relates to induction motor drives in general, and more particularly to adjustable frequency AC motor drives requiring torque information for regulation purpose.
Certain types of drives use a torque feedback signal for closed loop regulation. This is the case in particular of traction drives.
Instead of deriving directly a torque feedback signal, methods have been devised for the derivation of torque information from operating characteristics of the induction motor.
As compared to DC drives, motor drives using induction motors present added difficulties when adequate torque sensing is required. In DC drives, a direct and accurate measure of the generated electromagnetic torque is available in a straightforward manner through sampling of the armature current. In separately excited motors, the armature current is proportional to the torque. In series-excited motors, the torque is related to the armature current by a simple quadratic law. In induction motor drives, however, the torque, although related to the stator current, involves a much more complex relationship. The torque is proportional to only part of the stator current, namely to its component in phase with the "air gap voltage", a quantity which is internal to the motor but not measurable externally, and which depends upon the terminal voltage in accordance with a complex vectorial relationship that changes with speed, load and temperature. The torque is further related to the magnetizing current, which is another component of the stator current, namely one in quadrature with the air gap voltage.
Despite the complexity of the relationship between torque and current, the problem of obtaining an accurate torque signal by processing the motor variables has been solved in the prior art. One method uses "field-oriented" or "vector" control of the motor which leads to a signal representing the motor torque and considered as versatile as the torque feedback signal derived in DC drive. That torque representative signal can be used in a feedback torque control loop acting separately on the proper component of the stator current, in a manner that has been held to render an AC motor as controllable as a DC motor in a high performance DC drive. This field-oriented control approach is described in R. Gabriel and W. Leonhard "Microprocessor Control of Induction Motor" Conference Record of IEEE 1982 International Semiconductor Power Converter Conference pp. 385-396.
The amount of signal processing necessary to implement field-oriented control of an AC motor is considerable. It is felt that for a number of medium-performance motor drives, such as in traction, for instance, where fast dynamic response to the controls is not required, the complexity of field-oriented control is not justified.
The present invention is particularly suitable for moderate performance drives, and in particular where simplicity of control of induction motor is the primary goal, even if it is at the cost of some loss of performance in terms of speed of response. More specifically, the object of the present invention is to obtain through simple means, a feedback signal representing the torque generated by the motor, without being concerned with what particular use will be made of such signal. The invention is primarily concerned with sensing of the torque. The invention is also directed to control using such torque representative signal.